1. Field of the Invention
The present invention relates to an acceleration sensor with at least one magnetoresistive effect (MR) element and to a magnetic disk drive apparatus with the acceleration sensor.
2. Description of the Related Art
In a magnetic disk drive apparatus or a hard disk drive (HDD) apparatus assembled in a mobile equipment such as for example a walkabout personal computer, a mobile phone, a digital audio player and other mobile gear, an HDD apparatus used as a mobile storage itself or a removable HDD apparatus, in order to prevent a collision of a magnetic head with a hard disk surface due to drop impact, it is necessary to detect the instant at which the HDD apparatus falls before occurrence of the drop impact and to retract the magnetic head from the hard disk surface. Such instant of the falling can be detected from a slight change in the acceleration of gravity.
U.S. Pat. No. 6,131,457 (Japanese patent publication No. 11-352134A) discloses an acceleration sensor for detecting a small change in the acceleration of gravity. This acceleration sensor has a magnetic body including a mass point on an axis along a Z-axis, mounted to a vibrator having three-dimensional freedom, and four or more magnetoresistive effect (MR) detector elements positioned on an X-axis and a Y-axis with their centers located along a perimeter of a concentric circle around the origin point of the orthogonal coordinate axes. The sensor is thus capable of detecting each of acceleration in the direction of X-axis through a relative difference in output voltage between the two detector elements on the X-axis due to a vibration of the magnetic field, acceleration in the direction of Y-axis through a relative difference in output voltage between the two detector elements on the Y-axis due to a vibration of the magnetic field, and acceleration in the direction of the Z-axis through a sum total of the output voltages of all the detector elements.
However, because this known acceleration sensor disclosed in U.S. patent No. 6,131,457 has a structure with the permanent magnet mounted on the vibrator having three-dimensional freedom, this permanent magnet may be moved due to a force produced by an externally applied magnetic field. This movement of the permanent magnet will induce noises or detection errors of acceleration when precise detection of acceleration is performed. Such influence of the externally applied magnetic field will increase in case the acceleration sensor is downsized and thus the size of the permanent magnet decreases.
Also, because the known acceleration sensor uses an anisotropic MR (AMR) element with a single layer structure for each magnetic field detector element causing lower sensitivity of the magnetic field, it is difficult to detect acceleration with high sensitivity.